306 THE CHICK. 



arches, and divides the truncus arteriosus in such manner that 

 th e right division of the truncus arteriosus, an d consequen tly 

 the rip ;ht ventric le, from which this division arises, sends all its 

 blo od into the fifth pair of aortic arche s ; wh ile the left ventr icle 

 and l eft division of the truncus arteriosus co nduct blood to th e 

 t hird and fourth pairs of arches . It follows from this that the 

 supply of blood to the head and anterior part of the body is 

 derived from the left ventricle ; while the right ventricle supplies 

 the whole of the body behind the heart, as well as the yolk-sac 

 and allantois. 



About the seventh day the two divisions of the truncus 

 arteriosus separate completely from each other at their bases ; 

 the right branch, or pulmonary trunk, remaining in connection 

 with the right ventricle and the fifth pair of aortic arches ; and 

 the left branch, or systemic trunk, with the left ventricle and 

 the third and fourth pairs of aortic arches. 



The part o f the aorta connecting the dorsal ends of the th ird 

 and fourth pairs o f aortic arche s (Fig. 128) becom es very sle nder, 

 and I S fin ally obliterated altogether, while the branch of the 

 truncus arteriosus from which the thi rd aortic arch arises elon - 

 gate s very considera bly, and carries this arch forwards s ome 

 distance in front of the next^or fourth aortic arc h. The subclavian 

 arteries, suppiymg the fore-limbs, arise from the ventral ends of 

 the third aortic arches, during the third day ; they grow back- 

 wards, lying ventral to the other vessels, and reach the limbs 

 during the fourth or fifth day. 



The fourth pair of aortic arches is, from the fifth day 

 onwards, much the largest of the three persistent pairs. Jlhe^ 

 arches of the two sides of the body are at first of equal si ze : but 

 the arch of the right aide soon becomes much larger than that 

 of the left sid e ; and the l^erultim ately becomes oblite rated 

 along the greater part o f its length, while the arch of the right 

 side persists as the arch of the aorta in the adult bird. 



The jpnlmonary arteries appear, in t he walls of the l ungs, 

 about the middle of the third day, before the two hinder pairs 

 o f aortic arches are forme d. On the appearance of the fifth pair 

 of aortic arches the pulmonary arteries (Fig. 128, AP) become 

 connected with their ventral ends. E ach fi fth aortic arch t hus 

 c onsists of two pa rts : a proximal part running from the truncus 

 arteriosus, i.e. from the right ventricle, to the lung ; and a distal 



